Kill percentage flag used for -nomonsters and respawning

In the Doom source, the flag (used to calculate how many monsters are in a level, for the kill percentage) is also used to determine which monsters are hidden with the Parameter command line argument, and which monsters will respawn on Nightmare!. This has some unexpected consequences.

Lost souls
The lost soul monster had this flag removed for all engine versions beginning with Doom II's initial release. Likely, it was done to prevent the new pain elemental monster from skewing the kill percentage, or even making 100% impossible by ejecting lost souls through walls.

As a consequence to this, lost souls do not respawn. However, they are hidden with, due to a specific hack in : // don't spawn any monsters if -nomonsters if (nomonsters       && ( i == MT_SKULL || (mobjinfo[i].flags & MF_COUNTKILL)) ) {       return; }

Commander Keen
The Commander Keen object, which only appears in vanilla Doom on MAP32: Grosse (Doom II), is tagged with MF_COUNTKILL. Deathmatching with the causes Keen not to appear. Because killing the four Keens is needed to expose the end to the level (via tag 666), the level cannot be exited when using, unless one is also using the Parameter command.

Other special actions
has monsters that must be killed to access the exit, but has another exit reachable via a deathmatch spawnspot, and so can be exited in that mode even with. With some difficulty, the normal exit can also be accessed by rocket jumping or by zero press.